1. Field of the Invention
The present invention relates to a Si solar cell substrate. In particular, the invention relates to the structure of a solar cell substrate formed with thin film polysilicon.
2. Description of the Prior Art
Examples of current solar cells are Si, GaAs and CuInSe2, with the most accepted among the three being the Si solar cell. Advantages of Si solar cell include reasonable emission efficiency and production costs. Three types of Si solar cell are single crystal silicon, polysilicon, and amorphous silicon. Single crystal silicon and polysilicon exhibit higher efficiency, while amorphous solar cell, though cost-saving, is not as good, as stability is not satisfactory after emitting light. Recently, solar cells with thin film polysilicon has been developed to lower production costs for Si solar cells.
Production costs for solar cells with thin film polysilicon are quite low, because glass is used as substrate. In addition, they provide a large area and exhibit excellent semiconductor characteristics. Consequently, they have become a highly studied subject in this field.
Solar cell applications with thin film polysilicon are extensive, for instance, sensors, solar cells, thin film transistors etc. The most promising fabrication process at the moment is laser nucleation, followed by annealing of the nucleus amorphous silicon thin film to cause growth of silicon grains. A conventional solar cell substrate is shown in FIG. 1, where a polysilicon layer 6 is formed on a substrate 2 covered with a conductive layer 4. Using traditional method, grain size rarely exceeds 1 xcexcm, and is usually in the range of 0.01-10.5 xcexcm. However, larger grain size is preferable, as defect probability for electrons/holes to be trapped in the grain boundary is greatly reduced. By having greater sizes of Si grains, mobility and diffusion length are increased, thereby enhancing characteristics of the elements made thereof.
It is an object of the invention to provide a novel solar cell substrate with thin film polysilicon. By having a patternable thermal conducting layer and at least two different thermal conducting layers, polysilicon with larger grains is formed as a result induced by driving force for lateral growth.
In order to achieve the above objects, there is provided a solar cell substrate with thin film polysilicon, comprised of a substrate; a transparent conductive layer, formed on the substrate; a thermal isolation layer having inlaid conductive layers, formed on the transparent conductive layer; and a polysilicon layer, formed on the thermal isolation layer. The substrate is shown in FIG. 2, where a transparent conductive layer 4, thermal isolation layer 8 having inlaid conductive layer 7 and polysilicon layer 6 are sequentially formed on the substrate 2.
The substrate is conductive or non-conductive. The formation of the transparent conductive layer is omitted when using a conductive substrate. An example of a non-conductive substrate is glass.
Preferred thermal isolation layer is made of dielectric material, such as SiO2. The conductive layer inlaid in the thermal isolation layer is not restricted to specific material, any conductive material can be used.
Similarly, there is no limitation for the pattern of the conductive layer inlaid in the thermal isolation layer. It can be arranged in a matrix format.
According to the solar cell substrate with thin film polysilicon provided in the invention, conventional follow-up process is applicable to fabricate P type or N type of solar cell by forming PN of PIN junctions.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings, given by way of illustration only and thus not intended to be limitative of the present invention.